Using Plasmonics to Enhance Photocatalytic Water Splitting

Timespan: 
2010-2012
Principal Investigator: 

There has been a recent flurry of interest in combining plasmonics with photocatalysis. Plasmonics refers to the manipulation of light on sub-wavelength dimensions via surface plasmons in metallic nanostructures resulting in tremendous enhancements of the local field strength.  Perhaps the most well known example is Surface Enhanced Raman Scattering (SERS).  In the mid-1970s, it was found that Raman cross sections were enhanced by 6 orders of magnitude for molecules physisorbed to roughened silver surfaces.  By proper engineering the metal and dielectric nanostructures, i.e., choice of material and the particle’s size and shape, the resonant enhancement can be finely tuned to the spectral region of interest.

Photocatalysis refers to the process by which a chemical reaction is catalyzed, or sped up, in the presence of light.  There are two main areas of interest for photocatalysis.  

1.  The destruction of toxic organic molecules in our environment such as pesticides, volatile organic compounds (VOCs), and indoor pollutants, among other things.  

2.  The production of fuel using solar photons.  While both of these are important endeavors, our focus will be on employing photocatalysis for solar fuel production.

The question to be addressed is:  Will plasmonics allow and enhance photocatalytic water oxidation (also known as water splitting) using photons in the visible region of the solar spectrum?  If so, it will provide significantly higher efficiencies and will be a “game changing” breakthrough in the field of solar fuel production.

There are two steps required to produce a solar fuel:  1. Water oxidation provides electrons and protons that are used in subsequent reduction steps to produce a fuel, and 2. Reduction of the protons to produce H2(g), or reduction of CO2 to form CO (or formate, formic acid, methanol, etc.).  We will concentrate on the water oxidation part of the process:  2 H2O ® O2 + 4 e– + 4 H+.

Our goal is to utilize state-of-the-art amorphous electrocatalysts for water oxidation sensitized with dye molecules and plasmonic nanoparticles.  The dye sensitizers absorb photons in the visible region of the solar spectrum, and thereby can photo-inject requisite electrons for catalysis.  Electron injection by dye sensitizers is well known.  In fact, this is the basis for photographic film in which electrons are injected into AgBr, which is a wide bandgap semiconductor, via dye sensitizers.  The key difference between this and previous attempts of dye-sensitizing electrocatalysts is that we will employ plasmonic enhancement.

 
Hui Cao, Department of Applied Physics
Charles A. Schmuttenmaer, Department of Chemistry